This invention relates to a differential transmission for controlling the rotational speed of a pair of members and, more particularly, to an improvement in differential transmissions of the type which transmit internal resistance torque to the members, such as wheel axles.
Differential transmissions may be used in many fields where pairs of members are rotated. Typically, conventional differential transmissions are utilized to allow the driving wheels of a motor vehicle to rotate at different speeds while dividing the driving torque equally between the wheel axles and, hence, the wheels. This function is desirable such as when the vehicle is making a turn or cornering. During this turning condition, the outer wheel will rotate at a different, i.e., faster, speed than the inner wheel.
While the conventional differential transmissions perform the above function satisfactorily, they do have an important disadvantage. As is well known, when the vehicle is being driven, it is possible that one of the driving wheels will lose traction and spin, while the other wheel remains stationary, whereby the vehicle can become immobilized. This normally can occur when driving over low tractive surfaces such as mud, ice or snow. When the one wheel slips, it cannot absorb torque and no torque can be supplied to the other or stationary wheel. Conventional differential transmissions are deficient in that they do not provide at least a minimum resistance torque to eliminate wheel slippage on low tractive surfaces.
Other differential transmissions, known as limited slip differentials, have been developed to overcome the disadvantage of the above-noted conventional differential transmission. For example, one of these limited slip differentials, is described in U.S. Pat. No. 3,186,258, isued June 1, 1965. Essentially, limited slip differentials always provide at least a minimum amount of resistance torque internally of the differential in the form of internal friction. This internal friction is obtained by preloading or biasing the differential so that the torque absorption by either wheel cannot fall below a predetermined amount. This preloading or biasing provides internal torsional resistance to speed differentiation between the wheels, thereby insuring that a minimum torque is delivered to each wheel under all driving conditions.
A disadvantage with limited slip differentials is that the internal resistance or friction is present during all driving conditions irrespective of the speed differential between the wheels. When driving over normal or good tractive conditions in a straight path or when turning the vehicle on such normal tractive conditions, this internal friction is not required. Yet, during these driving conditions the vehicle must expend more energy to overcome this friction even though the friction is not required at this time. Also, undesirable heat and noise are produced as a result of this friction. Moreover, because the wheels of a vehicle having the conventional limited slip differential are frictionally coupled together through this differential at all times, the brakes will often lock up when applied, resulting in skidding.